1. Field of the Invention
The present invention relates generally to methods of modifying the properties of subterranean strata, and more particularly, to methods of increasing the resistance of the strata to shear failure.
2. Description of the Prior Art
Hydraulic cement compositions are commonly utilized in oil, gas and water well completion and remedial operations. For example, hydraulic cement compositions are used in primary cementing operations whereby a string of pipe such as casing is cemented in a well bore. In performing primary cementing, a hydraulic cement composition is pumped into the annular space between the walls of the well bore and the exterior of a string of pipe disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened substantially impermeable cement therein. The cement sheath physically supports and positions the pipe in the well bore and bonds the pipe to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
In some well locations, the subterranean strata into or through which wells are drilled have high permeabilities and low tensile strengths. As a result, the resistances of the strata to shear are low and they have low fracture gradients. When a well fluid such as a hydraulic cement composition is introduced into a well bore penetrating such a subterranean stratum, the hydrostatic pressure exerted on the walls of the well bore can potentially exceed the fracture gradient of the stratum and cause the formation of fractures into which the cement composition is lost. While light weight cement compositions have been developed and used, subterranean strata are still encountered which have fracture gradients too low for even the light weight cement compositions to be utilized without fractures and lost circulation problems occurring.
Thus, there are needs for methods of modifying the properties of subterranean strata penetrated by well bores to increase their resistance to shear failure, i.e., to increase the fracture gradients of the strata whereby well cements and other conventional well fluids can be utilized therein.